A protein kinase is an enzyme, which plays a key role in mediation of signal transduction via phosphorylation of a hydroxyl group present in a tyrosine, serine or threonine residue, and, thus, is deeply involved in the regulation of cell growth, differentiation, proliferation, etc.
As is well known, a balance between “on-states” and “off-states” of an intracellular signaling pathway is essential for maintenance of homeostasis of a cell. When a normal intracellular signaling pathway of, e.g., mostly continuation of “on-state” of intracellular signals is interrupted due to overexpression or mutation of a specific protein kinase, it may lead to an outbreak of various diseases such as cancer, inflammatory disease, metabolic disease and brain disease. It is estimated that human genome contains 518 protein kinases which constitute approximately 1.7% of all human genes [Manning et al., Science, 298, (2002), 1912]; and the protein kinases can be divided into tyrosine protein kinases (90 or more types) and serine/threonine protein kinases. The tyrosine protein kinases can be divided into receptor tyrosine kinases including 58 distinct kinases which can be further categorized into 20 subtypes, and cytoplasmic/non-receptor tyrosine kinases including 32 distinct kinases which can be further categorized into 10 subtypes. A receptor tyrosine kinase has a kinase domain on the surface where it can bind a growth factor, and an active site where phosphorylation of a tyrosine residue takes place. Binding of a growth factor to the extracellular domain of the receptor may cause the receptor tyrosine kinase to form a polymer, which may result in autophosphorylation of specific tyrosine residues in the cytoplasmic domain. This may trigger a cascade of events through phosphorylation of intracellular proteins that ultimately transmit the extracellular signal to the nucleus, thereby causing transcription and synthesis of various genes that may be involved in cell growth, differentiation, proliferation and the like.
Among the various cytoplasmic kinases, RAF is one of the kinases that participate in the linear Ras-RAF-MEK-ERK mitogen-activated protein kinase (MAPK) pathway initiated by a receptor protein kinase, which is activated by a growth factor [Solit, D. B. et al., Nature, 439, (2006), 358]. Currently, there are known three types of isoforms thereof, i.e., A-RAF, B-RAF and C-RAF (RAF-1) [Jansen H W, et al., EMBO J, 2, (1983), 1969; Marais R. et al., Cancer Surv, 27, (1996), 101]. Since abnormal activation in the MAPK pathway has been observed in approximately 30% of human cancer tissues and gene mutation of B-RAF and C-RAF showing aberrant activation has been confirmed in cancer tissues, it is generally accepted that RAF plays a very important role in the MAPK pathway of cancer tissues.
Accordingly, there have been suggested methods of using a compound having an inhibitory effect against abnormal activities of RAF kinases for treatment of cancer. Hence, a number of RAF and modified RAF kinase inhibitors are currently under development or being tested in ongoing clinical studies. Examples of such RAF kinase inhibitors include: sorafenib (Nexavar®, Bayer) which is used for treatment of liver cancer, vemurafenib (PLX-4032, RG7204, Roche) which has been recently approved for treatment of melanoma; and examples that are currently being tested in clinical trials include: regorafenib and RDEA119 by Bayer, RAF265 by Novartis, E3810 by Advan Chem, DCC2036 by Deciphera Pharma., CKI-27 by Chugai Pharma., RO-5126766 by Roche, etc.
However, efficacy of such drugs has been questioned when they are administered over a duration of time despite their good initial performance as drug resistance has been observed in some patients about 7 months after the initial administration of the drug.
It has been postulated that such degradation may be due to the drug resistance of B-RAF inhibitor which is caused by abnormal activation of MAPK pathway due to changes in RAF, activation of complementary signaling system among different RAF isoforms, or activation of various receptor kinases other than MAPK as a result of activation of different pathways of Ras, a key protein used in the signal-transducing cascade which consists of K-Ras, N-Ras and H-Ras subtypes.
One of the signaling pathways that the RAF kinases do not get involved is C-FMS (cellular feline McDonough sarcoma), also known as colony-stimulating factor-1 receptor (CSF-1R), which is a member of the family of genes originally isolated from the Susan McDonough strain of feline sarcoma viruses. FMS is a receptor for macrophage-colony-stimulating factor (M-CSF) encoded by the C-FMS proto-oncogene, which belongs to a class III RTK along with Kit, Flt-3 and PDGFR. It has been reported that FMS tyrosine kinase is involved in cancer metastasis.
Another example is a receptor protein tyrosine kinase called discoidin domain receptor (DDR), which is a subfamily of receptor tyrosine kinases that possess an extracellular domain related to the lectin discoidin. In case of animals such as humans, there are two types of DDR proteins, DDR1 type and DDR2 type, which have similar amino acid sequences and are encoded by different genes from each other. It has been reported that DDR proteins may be implicated in the process of cancer growth and metastasis. In addition, an upregulated expression of DDR has been observed in some tumor cells, along with a report that an upregulated expression of DDR raised expression of MMP-1 and MMP-2 which are known to be implicated in cancer growth. Thus, it is expected that inhibition of such kinases can lead to a therapeutic effect against various types of cancer. Therefore, a compound having an inhibitory activity against not only RAF, but also FMS, DDR1 and DDR2 kinases can be more useful for treatment of various cancers including resistant cancer, as compared with a conventional RAF kinase inhibitor.